Power semiconductor devices are fabricated by a series of batch processes in which wafers, typically of silicon or other compatible material, are processed to produce a particular type of power semiconductor chip. Each wafer contains a plurality of power semiconductor die or chips, typically of the same kind. As is known, chemical, thermal, photolithographic and mechanical operations are typically involved in the fabrication of the power semiconductor wafer. Because of variations across the wafer and across each individual chip caused by process variables or physical phenomena, however, not all chips on the wafer will meet the desired specifications for the chips. Some method of testing is employed to determine which chips alone or on any given wafer meet the specifications. Since the power semiconductors are designed to handle high currents and voltages, it is desirable to test the chip at its rated conditions. This is very difficult and presents several unique problems when performing the tests on the wafer or the chip before it is assembled into a package designed to handle the high power levels.
Power semiconductor chips are typically fabricated with one or more layers of metal interconnect on the surface of the chip which provide connecting paths to form the desired circuit. The metal interconnect layer or layers also provide a means to make connections to the power semiconductor chip when the chip is separated from the wafer and is assembled into a package or carrier. Interconnect points, typically called “bonding pads”, are formed by the metal interconnect and are arranged on the surface of the chip so as to allow bonding wires or other connecting means to be connected from the power semiconductor chip to its carrier or package. These same bonding pads, and others designed specifically for test purposes, can be used to make electrical contact to each individual chip for testing the electrical characteristics of the chips even while still joined together in wafer form or in chip form after separation from the wafer.
The yield of good chips on a wafer is defined as the percentage of good dies with respect to the total dies present on the wafer. Yield is the single most important cost factor in the production of power semiconductor devices. Each process and test step may be considered a potential yield loss point. The testing of each die on the wafer may result in yield loss not only from the manufacturing processes, but also from problems which can occur due to errors in testing operations. For example, during a probe testing operation, electrical contact is made to the bonding pads of each power semiconductor in order to electrically stimulate the circuit and to measure critical parameters. An array of fine wire probes, conductive bumps and/or fine beams formed on a card is/are aligned so as to correspond with the array of bonding pads and is/are used to mechanically and electrically contact the array of bonding pads. Typically, each die on the wafer is sequentially positioned and aligned under the array of probes, for example, and the wafer is moved up to allow contact of the respective probes onto the chip. Precision wafer movement stages allow each chip to be positioned under the probe array, brought into contact with the probe array, and tested. The chips on the wafer which do not pass the electrical test are marked by some method such as by applying a dot of ink or by storing their respective position on the wafer in computer memory for later recall.
In most cases, the interconnecting metal layer or layers of the power semiconductor chip are formed of aluminum or sometimes gold. These metals provide good processing characteristics and good electrical characteristics. However, these metals are also rather soft in comparison with the typical materials used for forming the probes on the card (referred to herein as a power semiconductor probe card). As a result, it is likely that damage to the bonding pad area or the probe card itself will occur if the probe card is not properly constructed, aligned, adjusted and/or utilized. For example, the tips of the probes are carefully adjusted for planarity to insure that all probes touch the respective bonding pads at relatively the same time. The probes also are adjusted to contact, e.g., touch down, accurately on each pad. After the probes initially contact the respective bonding pads, a proper amount of overdrive is maintained past the point of initial contact in order to provide a contacting force resulting in a consistent low resistance contact.
The tips of the probes themselves should be capable of providing low resistance contact between the probe and the bonding pads and should be free of contaminants that prevent good electrical contact. The contacting force or spring constant of the probe itself is also a parameter which should be considered in determining the ability of a probe to provide a proper contact. If a probe does not make good contact, attempting to pass high current to die will result in excessive heating at the contact point. This can very easily burn the tip of the probe causing further increase in resistance and further damage to the probe and possibly to the power semiconductor chip.
Various technologies have been used to produce probe cards for testing power semiconductors. The most common types are blade, epoxy ring and membrane technologies. A fourth type, which involves what is referred to a “buckling beam”, also has been used by some manufacturers. Blade technology is discussed in U.S. Pat. No. 4,161,692 for a “Probe Device for Power semiconductor Wafers”; U.S. Pat. No. 3,849,728 for a “Fixed Point Probe Card and an Assembly and Repair Fixture Therefor”; and U.S. Pat. No. 4,382,228 for a “Probes for Fixed Point Probe Cards”. Epoxy ring technology is discussed in U.S. Pat. No. 3,835,381 for a “Probe Card Including a Multiplicity of Probe Contacts and Methods of Making”; U.S. Pat. No. 3,905,008 for a “Microelectronic Test Probe Card Including a Multiplicity of Probe Contacts and Method of Making Same”; U.S. Pat. No. 4,599,559 for “Test Probe Assembly for IC Chips”; and U.S. Pat. No. 4,757,256 for a “High Density Probe Card”. Buckling beam technology is discussed in U.S. Pat. No. 4,554,506 for a “Modular Test Probe”; and U.S. Pat. No. 4,843,315 for a “Contact Probe Arrangement for Electrically Connecting a Test System to the Contact Pads of a Device to be Tested”.
The most commonly used type of technology to produce power semiconductor probe cards is epoxy ring technology, although the other technologies are similar. In the construction of an epoxy ring type probe card, a sheet of Mylar is punched or drilled with a series of holes in the same array pattern as the bonding pad locations on the chip. The holes are sized to accept the tip of each probe and hold the tip in position during construction of the card. These holes are typically 0.003 inch to 0.005 inch in diameter. Each probe is made from a length of spring wire which is tapered to a point at one end and bent down at a steep angle to form a probe tip. Each probe tip is placed in a corresponding hole in the Mylar sheet. The other end of each spring wire probe is arrayed in a generally circular pattern with those of the other probes and is secured in place by a ring of epoxy or another suitable material. The ends protrude through the epoxy in order to be soldered to a circuit board which forms the probe card. After the probes are soldered to the circuit board, the probe tips are sanded to provide relatively flat probe tips positioned in a relatively planar array.
Contact resistance of the probe tips can be measured using conventional techniques for the measurement of low resistances. A typical method would be to bring the probe tip into contact with a conducting metal surface and measure the resistance of the resulting interface. The type of metal used for the contact plate is typically gold, nickel or Rhodium and some differences will be observed between the resistance measured by these conventional methods and the actual resistance observed when the probe is contacting bonding pads formed using aluminum metallization, for example, on the power semiconductor chip.
Furthermore, since the aluminum is rather soft in comparison with the probe tip material, the tip of the probe will tend to protrude or “dig” into the aluminum and make contact over a much larger surface area of the tip as compared to on the harder gold surface. The angle of the probe tip relative to the bonding pad is such that a scrubbing motion is created when the tip is driven against the pad. In the case of a bonding pad made of soft aluminum, this creates a scrub mark corresponding to the path of the probe tip on the pad. The material used for the probe tip is critical to insure a low contact resistance between the probe tip and the pad. The typical material used for power semiconductor probing is beryllium copper although some probe cards use standard tungsten probes for lower cost.
Since a single probe can handle only a limited amount of current, multiple probe connections are typically made to each pad of the power semiconductor chip. It is standard practice to use more probes on a pad than would typically be required to carry the desired current since it is known that some probes will have a higher resistance and not be able to carry their share of the load. There is a trade-off between how many probes can be feasibly and economically placed on each pad and desired redundancy of the connection. The typical problem with this technique is that there is no indication of a contact problem in the test other than a loss of yield because the chip does not meet its specifications. Without some other indicator, the tester cannot determine whether it is the chip or the probe card which is causing the problem. Testing may proceed for some time before it can be determined that the probe contact is bad and the probe card should be serviced or replaced. Since the cost of the tester, wafer prober, facility, etc., is very high, this can be very costly and should be avoided if possible. Also, continuing to pass current though probes with high contact resistance will very likely burn the probe tips and may damage the aluminum pads by creating defects in the pad metallization.
The same conditions exist on integrated circuit chips which require high levels of supply and ground currents such as a high performance microprocessor. These chips are normally constructed with multiple supply and ground pads to share the current. As in power semiconductor chips, if some of the probe tips lose contact with their respective pads, the remaining probe tips share the total current and operate at a higher level. At some point this current level may become too high and exceed the specified probe tip current carrying capacity.
A second problem is that the best probes, i.e., those with the lowest resistance will tend to have the highest current. This may over stress these “good probes” and cause them to fail thus compounding the problem. It is desirable to insure that the current is shared relatively equally among the probes and does not exceed the specified maximum for any single probe. The standard method of merely paralleling multiple probes on a given pad of a power semiconductor chip or on the multiple pads of an integrated circuit chip does not accomplish this goal since the contact resistance of the probe tips to the pad can vary widely.